Nitrogen cycle transformations in sewage treatment Arrange the sequence of nitrogen transformations occurring from organic nitrogen to gaseous nitrogen under typical biological processes.

Introduction / Context:
In wastewater treatment, nitrogen is transformed through a well-known sequence: ammonification, nitrification, and denitrification. Understanding the order is essential to design and operate biological nutrient removal systems and to interpret process monitoring data (e.g., ammonia, nitrite, nitrate concentrations).

Given Data / Assumptions:

• Organic nitrogen in sewage is converted by microbes.
• Oxic and anoxic environments are available in treatment stages.
• Final end product of denitrification is predominantly nitrogen gas.

Concept / Approach:

First, organics undergo ammonification releasing ammonia (NH3/NH4+). Next, nitrifying bacteria oxidize ammonia to nitrite (NO2−) and then to nitrate (NO3−). Finally, under anoxic conditions, denitrifiers reduce nitrate to nitrogen gas (N2), which is released to the atmosphere. This is the textbook nitrogen cycle within treatment systems.

Step-by-Step Solution:

Verification / Alternative check:

Process monitoring in BNR plants shows decreasing ammonia with corresponding increases in nitrite/nitrate in aeration, followed by nitrate decrease and nitrogen gas release in anoxic tanks.

Why Other Options Are Wrong:

(b) and (c) invert the sequence. (d) is nonsensical, repeating nitrates without proper steps. (e) is unnecessary since a correct option exists.

Common Pitfalls:

Confusing nitrification (strictly aerobic) with denitrification (anoxic), or assuming nitrite is always detectable—it is often quickly oxidized to nitrate in well-aerated systems.

Final Answer:

Liberation of ammonia → formation of nitrites → formation of nitrates → liberation of nitrogen